Surface acoustic wave functional device

ABSTRACT

The surface acoustic wave functional element comprises a semiconductor layer provided on a piezoelectric substrate or a piezoelectric film substrate and makes use of interaction between a surface acoustic wave propagating on the substrate and electrons in the substrate layer, but has the semiconductor layer disposed outside above the propagation path for propagating a surface acoustic wave, comprises a plurality of grating electrodes perpendicularly above and to the propagation path and moreover the semiconductor layer comprises an active layer and a buffer layer lattice-matching thereto. By use of this surface acoustic wave functional element, a surface acoustic wave amplifier capable of providing a high amplification gain at a practical low voltage, a surface acoustic wave convolver having a higher efficiency than ever or the like are offered.

TECHNICAL FIELD

The present invention relates to a surface acoustic wave functionalelement, such as surface acoustic wave amplifier or surface acousticwave convolver, by use of interaction between a surface acoustic wavepropagating through the piezoelectric substrate and electrons in thesemiconductor.

BACKGROUND ART

Known as one conventional functional element by use of interactionbetween a surface acoustic wave and electrons in the semiconductor is asurface acoustic wave functional element of such a structure as toperform an interaction over the propagation path all in width of asurface acoustic wave. With respect to a surface acoustic wave amplifieras one example of surface acoustic wave functional element, for example,three structures have been proposed: direct type amplifier (FIG. 2),separate type amplifier (FIG. 3) and monolithic type amplifier (FIG. 4).The first direct type amplifier is of a structure that a piezoelectricsemiconductor substrate 11 such as CdS or GaAs in possession of bothpiezoelectricity and semiconductivity is used to install thereon aninput electrode 4, an output electrode 5 and an electrode 8 for applyinga DC electric field to the piezoelectric semiconductor substrate 11,thereby amplifying a surface acoustic wave. However, no piezoelectricsemiconductor in possession of both large piezoelectricity and largeelectron mobility has been found thus far. The second separate typeamplifier is an amplifier of a structure with an input electrode 4 andan output electrode 5 provided on a piezoelectric substrate 1 of a largepiezoelectricity and a semiconductor 12 of a large electron mobilitydisposed via a gap 13 as well. In this type of amplifiers, theamplification gain is largely affected by the flatness of the surface ofthe semiconductor and the piezoelectric substrate and by the magnitudeof the gap. To obtain the amplification gain equal to practical use, itis required to make the gap as small as possible and keep it constantall over the operating region, so that there is an extreme difficulty inindustrial production. On the other hand, the third monolithic typeamplifier is an amplifier of a structure with an input electrode 4 andan output electrode 5 provided on a piezoelectric substrate 1 and asemiconductor 12 formed via a dielectric layer 14 rather than a gap 13as well. According to 1970s studies by Yamanouchi and others (K.Yamanouchi et al., Proceedings of the IEEE, 75, p. 726 (1975)), anelectron mobility for InSb of 1600 cm²/Vs has been obtained in astructure with SiO coated onto the LiNbO₃ substrate and a 50 nm thickInSb film deposited thereon and a gain of 40 dB has been obtained at acentral frequency of 195 MHz under application of an extremely high DCvoltage of 1100 V in a surface acoustic wave amplifier using this film.Since no good film quality of InSb was obtained, however, there has beena problem of too high driving voltage and too small amplification gainat a low voltage in consideration of applications to an actual portableapparatus.

Next, a surface acoustic wave convolver can be referred to as anotherapplication by use of interaction between a surface acoustic wave andelectrons in a semiconductor. At present, surface acoustic waveconvolvers arrest attention greatly as correlators for CDMA (CodeDivision Multiple Access) scheme of spread spectral communication. Sinceformer times, digital LSI and analog LSI have been examined as CDMAcorrelators, but either of them was extremely large in powerconsumption, thus forming an extremely large barrier againstapplications to a handy device requiring a low power consumption. Thus,a surface acoustic wave convolver of zero consumed power in principlebegins to be examined to practical use with advantages taken of lowpower consumption and no need for synchronism. In studies of a surfaceacoustic wave convolver, a convolution output of −59 dBm has beenobtained at the system of InSb/LiNbO₃, for example according to K.Yamanouchi, S. Mitsui and K. Shibayama, IEEE MTT-S Intern. MicrowaveSymp. Digest, p. 31 (1980).

To ensure applicabilities of a monolithic type amplifier to actualportable telephone or the like, however, it is required to obtain abetter amplification gain at a practically low voltage of at least 9 Vor lower and to implement it in a feasible process as well. In otherwords, a lower voltage than the former technique by two factors or moremust be intended. Besides, as regards a surface acoustic wave convolver,a still greater efficiency must be attained.

In a former structure of surface acoustic wave functional elements, ithas been required to lessen the thickness of a semiconductor filmgreatly in using such a semiconductor as InSb of a large electronmobility to match the electric impedance of a surface acoustic wave withthat of the semiconductor. With a thin film thickness, however, thesemiconductor film is poor in crystallinity and becomes smaller inelectron mobility, so that no functional element better incharacteristics has been obtained.

Besides, in convolvers, because of a small thickness of thesemiconductor layer, there has been a problem for a method of taking outan output in a direction of thickness that no high efficiency isobtained and moreover a problem that the sheet resistance reduces, thusleading to a short circuit in the electric field of a surface acousticwave has occurred for a greater thickness of the semiconductor layer.Furthermore, in a structure of forming a semiconductor layer above thepropagation path, the loss of a surface acoustic wave has increased,thus causing a decrease in amplification gain and a lowering ofefficiency.

Still further, no attention whatever has been paid to the presence of abuffer layer, the position of a grounded electrode and interaction inshape of a strip electrode.

DISCLOSURE OF THE INVENTION

It is one object of the present invention to provide a surface acousticwave functional element easy of industrial production, having asemiconductor film good in film quality as the active layer with thesemiconductor so arranged that an interaction between a surface acousticwave and the semiconductor occurs sufficiently.

As a result of intensive examination for solving the above task, theinventors implemented a surface acoustic wave amplifier of greatamplification characteristics and a surface acoustic wave convolver ofan extremely high efficiency under a low voltage wherein thesemiconductor layer was improved in crystallinity by inserting betweenthe piezoelectric substrate and an active layer a buffer layerlattice-matched to the active layer and further an interaction isenabled to occur in the semiconductor layer by disposing a semiconductorlayer beside the propagation path and using a grating electrode toconvey the electric field of a surface acoustic wave to thesemiconductor layer.

An extremely great amplification characteristic and an extremely highefficiency of convolution output under low voltage was implemented inthe present invention 1) because an extremely good active layer could beformed on the piezoelectric substrate by inserting a buffer layer in thegrowth of a semiconductor layer, 2) because the absence of asemiconductor layer on the propagation path of the piezoelectricsubstrate could minimize the loss of a surface acoustic wave, 3) becausethe electrode width and the electrode interval in grating electrodesdisposed on the propagation path were selected so as to suppress thereflection and 4) because the efficiency of interaction between asurface acoustic wave and electrons could be improved by the formationof an output electrode alternating the grating electrode for an electricsurface wave convolver.

Here, the active layer means a layer in which electrons to interact withthe surface acoustic wave propagated.

By properly selecting the relatively positional relation among thesemiconductor layer (active layer) improved in film quality as a resultof such insertion of a buffer layer, the grating electrode and theoutput electrode, the above object was attained.

Namely, 1) a surface acoustic wave functional element according to thepresent invention comprises an input electrode, an output electrode anda semiconductor layer provided on the piezoelectric substrate whereinthe above semiconductor layer is situated outside above the propagationpath of a surface acoustic wave propagated from the input electrode, thesemiconductor layer is composed of an active layer and a buffer layeraligned thereto, and further comprises a plurality of grating electrodesplaced on the above propagation path perpendicularly to the propagationdirection and in a greater width than that of the propagation path.

2) In the 1) mentioned above, one-end portions of the above ratingelectrodes may be formed on the above semiconductor layer.

3) In the 1) or 2) mentioned above, the functional element may comprisea plurality of grating electrodes with their width L satisfying L=λ/3n(n: positive integer) and their inter-electrode space S satisfyingS=λ/3n (n: positive integer) for the wavelength λ of a surface acousticwave propagating through the above propagation path.

4) In the 3) mentioned above, it is preferable that the width L of theabove grating electrodes satisfies λ/8≦L≦λ and the space S satisfiesλ/8≦S≦λ.

5) In the 3) mentioned above, it is preferable that the width L of theabove grating electrodes satisfies L=λ/6 and the inter-electrode space Ssatisfies λ/6.

6) The surface acoustic wave functional element described in any of theabove 1) to 5), may further comprise an electrode for applying a DCelectric field to the above semiconductor layer.

7) The surface acoustic wave functional element described in any of theabove 1) to 5), is characterized in that two input signals propagatingfrom a reference signal input electrode and the above input electrode issubjected to convolution with the above output electrode employed forthe reference signal input electrode.

8) In the above 7), the functional element may comprise an outputelectrode so arranged as to cross the above grating electrode and becomeequal in potential thereto.

9) In the above 8), the above output electrode may be formed above thesemiconductor layer.

10) In the above 8), the above output electrode may be formed below thesemiconductor layer.

11) In any of the above 7) to 9), the functional element may comprise auniform output electrode at the bottom of the above semiconductor layer.

12) In any of the above 7) to 11), the functional element may comprise auniform ground connection output electrode at the bottom of the abovepiezoelectric substrate.

13) In any of the above 8) to 12), a grating electrode above thepropagation path may differ in electrode period from a grating electrodeformed in alternating above or below the semiconductor layer and theoutput electrode.

14) Besides, the surface acoustic wave functional element of the presentinvention, comprising a grating electrode and a semiconductor layerprovided on a piezoelectric substrate or on a piezoelectric filmsubstrate, is characterized in that the above semiconductor layer issituated outside above the propagation path for a surface acoustic wavepropagating, a plurality of grating electrodes are formedperpendicularly to the propagating direction above the semiconductorlayer, a strip dielectric film is formed at the top of gratingelectrodes in other part than the semiconductor layer and an outputelectrode is formed on the relevant dielectric film.

15) In the above 14), the above strip dielectric films may be formed atthe top and the bottom of the grating electrode.

16) In the above 15), an output electrode may be formed below the stripdielectric film formed at the bottom of the above grating electrode.

17) In any of the above 14) to 16), a uniform output electrode may beformed at the bottom of the above semiconductor layer.

18) In any of the above 14) to 16), the above grating electrode may beformed below the semiconductor layer.

19) In the above 18), a uniform output electrode may be formed at thetop of the above semiconductor layer.

20) In the above 14) to 19), the above grating electrodes have astructure of lengths in width alternately varied in an appropriatecombination and a strip dielectric film may be formed at respective oneends of alternate grating electrodes.

21) In the above 14) to 19), output electrodes are formed so as to crossthe above grating electrodes and may be connected so as to become thesame potential.

22) Furthermore, the surface acoustic wave functional element of thepresent invention, comprising a semiconductor layer, grating electrodesand an output electrodes provided on a piezoelectric substrate or on apiezoelectric film substrate, is characterized in that the abovesemiconductor layer is situated outside above the propagation path for asurface acoustic wave propagating, a plurality of grating electrodes areformed perpendicularly to the propagating direction above thesemiconductor layer, the above output electrodes are formed across theabove grating electrodes so as to become the same potential and groundoutput electrodes, formed in a narrower width than that of thepropagation path on and across the grating electrode portions opposed tothe above semiconductor layer, are connected to a common electrode.

23) In the above 22), the above grating electrodes and the above outputelectrodes may be formed below the semiconductor layer.

24) In any of the above 21) to 23), the above output electrodes may beformed over the semiconductor layer part to the propagation path.

25) The surface acoustic wave functional element of the presentinvention, comprising a semiconductor layer, grating electrodes and anoutput electrodes provided on a piezoelectric substrate or on apiezoelectric film substrate, is characterized in that the above gratingelectrodes are formed up to halfway on the top or bottom of thesemiconductor layer and a uniform output electrode having a gap with theends of the above grating electrodes is formed.

26) The surface acoustic wave functional element described in any of theabove 14) to 25), is characterized in that the grating electrodes abovethe propagation path differ in electrode period from those formed aboveor below the semiconductor layer, or above or below the ground outputelectrode part or the strip dielectric film.

27) In any of the above 14) to 26), it is preferable that the width L ofthe above grating electrodes satisfies λ/8≦L≦λ and the space S betweenthe grating electrodes satisfies λ/8≦S≦λ.

28) In any of the above 21) to 26), it is preferable that the electrodewidth L of the alternating portion of the above grating electrodes withan output electrode or a ground output electrode satisfies λ/16≦L≦λ/2and the inter-electrode space S satisfies λ/16≦S≦λ/2.

29) In any of the above 14) to 29), it is preferable that the width L ofthe above grating electrodes is λ/6 and the space S between the gratingelectrodes is λ/6.

30) In any of the above 14) to 26), the above semiconductor layer may becomposed of an active layer and a buffer layer lattice-matching thereto.

31) In any of the above 1) to 30), the ratio of the width W of thepropagation path for a surface acoustic wave to the width a of thesemiconductor layer is preferably determined in such a manner as toalmost equate the electric wave impedance of a surface acoustic wave tothat of the semiconductor layer.

32) With the surface acoustic wave functional element described in anyof the above 1) to 31), the ratio of the width W of the abovepropagation path to the width a of the above semiconductor layerpreferably satisfies W/a>1.

33) With the surface acoustic wave functional element described in anyof the above 1) to 32), the ratio of the width W of the abovepropagation path to the width a of the above semiconductor layerpreferably satisfies W/a=8-10.

34) With the surface acoustic wave functional element described in anyof the above 14) to 33), a semiconductor selected from a groupcomprising Si, InAs, InSb, GaAs and InP may be employed for asemiconductor layer.

35) With the surface acoustic wave functional element described in anyof the above 1) to 34), a substrate selected from a group comprisingLiNbO₃ single-crystal substrate, LiTaO₃ single-crystal substrate andKNbO₃ single-crystal substrate may be employed for the abovepiezoelectric substrate.

36) With the surface acoustic wave functional element described in anyof the above 1) to 34), a piezoelectric film substrate formed using afilm selected from a group comprising LiNbO₃ film, LiTaO₃ film, KNbO₃film, PZT film and PbTiO₃ film may be employed for the abovepiezoelectric substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of a surface acoustic wavefunctional element according to one embodiment of the present invention;

FIG. 2 is a schematic sectional view of a former direct type amplifier;

FIG. 3 is a schematic sectional view of a former separate typeamplifier;

FIG. 4 is a schematic sectional view of a former monolithic typeamplifier;

FIG. 5 is a schematic perspective view showing the enlargedsemiconductor layer and grating electrode section of a surface acousticwave functional element, comprising a semiconductor layer, composed of abuffer layer and an active layer, and grating electrodes, according toone embodiment of the present invention;

FIG. 6 is a schematic perspective view of a former surface acoustic waveamplifier of a structure with a semiconductor layer provided above apropagation path;

FIG. 7 is a schematic perspective view of the semiconductor layer, thedielectric layer and the grating electrode section in an enlargedillustration of a surface acoustic wave functional element with adielectric layer inserted between the buffer layer and the piezoelectricsubstrate, according to one embodiment of the present invention;

FIG. 8 is a schematic perspective view of the semiconductor layer andthe grating electrode section in an enlarged illustration of a surfaceacoustic wave functional element with three buffer layers, according toone embodiment of the present invention;

FIG. 9 is a schematic perspective view of a surface acoustic wavefunctional element using a piezoelectric film as the substrate,according to one embodiment of the present invention;

FIG. 10 is a sectional view of a surface acoustic wave functionalelement of a structure with a uniform output electrode below the bufferlayer, according to one embodiment of the present invention;

FIG. 11 is a schematic perspective view of a former surface acousticwave convolver of a structure with a semiconductor layer provided abovethe propagation path;

FIG. 12 is a schematic perspective view of a surface acoustic wavefunctional element of a structure with grating electrodes formed belowthe buffer layer, according to one embodiment of the present invention;

FIG. 13 is a schematic perspective view of a surface acoustic wavefunctional element of a structure with grating electrodes and outputelectrodes alternating each other on the active layer, according to oneembodiment of the present invention;

FIG. 14 is a schematic perspective view of a surface acoustic wavefunctional element with a semiconductor layer, grating electrodes, anoutput electrode alternating the grating electrodes and a ground outputelectrode formed, according to one embodiment of the present invention;

FIG. 15 is a schematic perspective view of a surface acoustic wavefunctional element with a semiconductor layer, grating electrodes, anoutput electrode alternating the grating electrodes and a ground outputelectrode formed so that the period of the alternating portion differsfrom that above the propagation path, according to one embodiment of thepresent invention;

FIG. 16 is an enlarged illustration of the alternating portion ofgrating electrodes and an output electrode and that of gratingelectrodes and a ground output electrode in a surface acoustic wavefunctional element according to one embodiment of the present invention;

FIG. 17 is a waveform diagram showing the convolver output waveformobtained actually from a surface acoustic wave functional elementaccording to one embodiment of the present invention;

FIG. 18 is a schematic perspective view of a surface acoustic wavefunctional element with the alternating portion of grating electrodesand an output electrode formed over the active layer to the propagationpath, according to one embodiment of the present invention;

FIG. 19 is a schematic perspective view of a surface acoustic wavefunctional element with deformed grating electrodes formed on the activelayer and an output electrode and the ground output electrodealternating the grating electrodes formed, according to one embodimentof the present invention;

FIG. 20 is a schematic perspective view of a surface acoustic wavefunctional element comprising grating electrodes formed on the activelayer and comprising an output electrode formed so as to cross thegrating electrodes above the propagation path, according to oneembodiment of the present invention;

FIG. 21A is a schematic perspective view of a surface acoustic wavefunctional element of a structure with grating electrodes formed on theactive layer and a strip dielectric film formed at the grating electrodesections opposed to the semiconductor layer, according to one embodimentof the present invention;

FIG. 21B is a sectional view taken along the line X-X′ of FIG. 21A;

FIG. 22 is a schematic perspective view of a surface acoustic wavefunctional element of a structure with grating electrodes and an outputelectrode alternating them formed above the semiconductor layer and astrip dielectric film formed at the grating electrode sections opposedto the semiconductor layer, according to one embodiment of the presentinvention;

FIG. 23 is a schematic perspective view of a surface acoustic wavefunctional element of a structure with grating electrodes of alternatelyvaried lengths in an appropriate combination formed below thesemiconductor layer and a strip dielectric film formed at the respectiveone ends of alternate grating electrodes, according to one embodiment ofthe resent invention; and

FIG. 24 is a schematic perspective view of a surface acoustic wavefunctional element of a structure with a uniform output electrode formedat the bottom of the semiconductor layer so as to have a gap to one endof each grating electrode, according to one embodiment of the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in further detail.FIG. 1 shows a surface acoustic wave element on which the presentinvention centers, comprising a piezoelectric substrate 1, a bufferlayer 2, an active layer 3, an input electrode 4, an output electrode 5,an electrode 6 for applying a DC electric field to a semiconductor and agrating electrode 7.

A piezoelectric substrate according to the present invention may be asingle-crystalline substrate of piezoelectrics or those with apiezoelectric film formed on the substrate. To acquire a betterperformance in the surface acoustic wave functional element of thepresent invention, it is preferred to use a piezoelectric substrategreater in electro-mechanical coupling coefficient. Forsingle-crystalline substrate of piezoelectrics, an oxide piezoelectricsubstrate, such as LiNbO₃, LiTaO₃, Li₂B₄O₇ or KNbO₃, is preferably used.Besides, it is preferred to use a substrate of cut plane, such as 64degree Y cut, 41 degree Y cut, 128 degree Y cut, Y cut, X cut or Z cutof LiNbO₃ or 36 degree Y cut of LiTaO₃. Piezoelectric film substratesare those with a piezoelectric film formed on a single-crystallinesubstrate of sapphire, Si, GaAs or the like, while examples of thin-filmmaterials used preferably for piezoelectric film are ZnO, LiNbO₃,LiTaO₃, KNbO₃, PZT, PbTiO₃, BaTiO₃ and Li₂B₄O₇. Besides, a dielectricfilm of SiO, SiO₂ or the like may be inserted between asingle-crystalline substrate of sapphire, Si, GaAs or the like and theabove piezoelectric film. Furthermore, as a piezoelectric filmsubstrate, a multilayered film like films of different sorts among theabove piezoelectric films stacked over each other may be formed on asingle-crystalline substrate of sapphire, Si, GaAs or the like. Forexample, a multilayered film made up of LiNbO₃ and LiTaO₃ is a preferredexample.

For active layers, those of a large electron mobility are preferablyused to promote the performance of a surface acoustic wave functionalelement. Preferred examples include GaAs, InSb, InAs and PbTe. Besides,not only binary systems but ternary or quaternary mixed crystals made ofcombinations thereof are preferably used. For example, In_(x)Ga_(1-x)As,In_(x)Ga_(1-x)Sb, InAs_(y)Sb_(1-y) and GaAs_(y)Sb_(1-y) are examples ofternary mixed crystals, while In_(x)Ga_(1-x)As_(y)Sb_(1-y) is an exampleof quaternary mixed crystal. To acquire a high electron mobility ofactive layers, as regards x in In_(x)Ga_(1-x)M (M: V group semiconductorsuch as As or Sb) as the composition of active layers, a high electronmobility is obtainable generally within 0≦x≦1.0, but 0.5≦x≦1.0 ispreferable and 0.8≦x≦1.0 is a more preferred range. As regards y inRAs_(y)Sb_(1-y) (R: III group semiconductor such as In or Ga), a higherelectron mobility is obtainable within a range of 0≦y≦1.0 and 0≦y≦0.5 ispreferable.

Besides, the thickness hi of an active layer is preferably 5 μm or less,more preferably 1 μm or less and still more preferably 0.8 μm or less toimplement a low carrier density of the active layer and allow a surfaceacoustic wave and electrons to efficiently interact with each other andmoreover to prevent the breaking of wire in a grating electrode in thecase of forming the grating electrode on the semiconductor layer.Besides, a value of resistance in the active layer is preferably 10 Ω orhigher, more preferably 50 Ω or higher and still more preferably 100 Ωor higher.

The piezoelectric substrate and the active layer are quite differentboth in crystal structure and in lattice constant from each other. Forexample, LiNbO₃ of the piezoelectric substrate is of trigonal system,whereas InSb of the active layer is of zinc blend system. The latticeconstant also differs by more than 25%. In consequence, even if anattempt is made to allow InSb to grow on the LiNbO₃ substrate as itstands, many defects appear and no good film quality is obtained. Thus,the present invention found that use of a compound semiconductoridentical in crystal structure and relatively near in lattice constantto InSb as a buffer layer permits an active layer good in film qualityto be implemented. Furthermore, the buffer layer of the presentinvention is characterized in having a high resistance and forming nocurrent leakage layer in the interface with the piezoelectric substrate.Besides, in the buffer layer of the present invention, the electricfield of a surface acoustic wave was found to be characterized in hardlyattenuating. Furthermore, from the viewpoint of crystal growthtechnology, the compound semiconductor constituting the buffer layer ofthe present invention was confirmed to be extremely rapid in crystalrelaxation and to begin to grow in a slight thickness at a structure andlattice constant peculiar to the relevant compound semiconductor, thuspermitting an underlayer for nuclear formation of the active layer toform.

As buffer layers according to the present invention, binary systems suchas AlSb, ZnTe and CdTe and ternary systems such as AlGaSb, AlAsSb andAlInSb and quaternary systems such as AlGaAsSb, AlInAsSb, AlInGaSb,AlInPSb and AlGaPSb are preferred examples. Furthermore, in determiningthe composition of the above buffer layers of ternary systems orgreater, a larger electron mobility of active layer can be implementedby adjustment to a composition identical or near in lattice constant tothe crystal constituting the active layer. The lattice matching referredto as in the present invention indicates the same in crystal structureand the nearness in lattice constant. Here, the nearness in latticeconstant means that the difference in lattice constant between thecrystal constituting an active layer and that constituting a bufferlayer is within ±10%, preferably within ±7% and more preferably within±5%. Besides, for a more efficient interaction between a surfaceacoustic wave and electrons, the smaller thickness of the buffer layerbecomes the better. Namely, the thickness h2 of the buffer layer rangespreferably 5 nm≦h2≦3000 nm, more preferably 10 nm≦h2≦2000 nm and stillmore preferably 20 nm≦h2≦1000 nm. Besides, the above buffer layer mustbe electrically insulated from electrons in the active layer. Namely, itis advisable that the resistance of the buffer layer is made at least5-10 times larger, preferably 100 times larger and more preferably 1000times larger than that of the active layer.

Besides, the buffer layer of the present invention may be a multilayerof two types or more of semiconductor films.

In the case of a multilayer of two types or more of buffer layers, sinceit is necessary that only the buffer layer in contact with the activelayer is high in resistance, such conductive materials as InSb or GaAsSbalso can be employed for the buffer layer except the above examples ofbuffer layers. Besides, two sorts of buffer layers among the abovebuffer layers may be alternately stacked to make a super latticestructure. It contributes to the promotion of film characteristics inthe active buffer layer that the top layer among the buffer layersstacked with two sorts or more of films is made near in lattice constantto the active layer. Besides, as with the conditions of the above bufferlayer, the smaller the thickness of a multilayered buffer layer is, themore advantageous for preventing the wire breaking of the gratingelectrode.

In the present invention, a dielectric layer may be inserted between thepiezoelectric substrate and the buffer layer. This dielectric layer isused in some cases to protect the piezoelectric substrate and thesemiconductor film formed thereon. Examples used for the dielectriclayer are SiO, SiO₂, silicon nitride, CeO₂, CaF₂, BaF₂, SrF₂, TiO₂,Y₂O₃, ZrO₂, MgO, Al₂O₃ and Ta₂O₅. The dielectric layer becomes thebetter for the smaller and is preferably 200 nm thick or less and morepreferably 100 nm thick or less.

For the strip dielectric film of the present invention, materials of theabove dielectric layer can be employed similarly. Incidentally, thestrip dielectric film is formed for an efficient takeout of convolutionoutput. Namely, the convolution output due to an interaction between asurface acoustic wave and electrons in the semiconductor layer and theconvolution output above the propagation path are enabled to be addedtogether via the grating electrode.

With respect to materials of an electrode on the piezoelectric substrateand a grating electrode above the propagation path, there is inparticular no restriction, but electrodes of Al, Au, Pt, Cu, Al-Tialloy, Al-Cu alloy and a multilayered electrode of Al and Ti, forexample, are preferably employed.

With respect to materials used in electrodes for application of a DCelectric field to the semiconductor layer, there is in particular norestriction, but Al, Au, Ni/Au, Ti/Au, Cu/Ni/Au and AuGe/Ni/Au arepreferably employed.

A surface acoustic wave propagating through the propagation path isnormally reflected from the grating electrode, but minimizing thisreflection also leads to an improvement in amplification gain andefficiency. Thus, the grating electrode of the present invention isformed at an electrode width and electrode space as efficientlyconveying the electric field of a surface acoustic wave to thesemiconductor layer and minimizing the reflection. Namely, the electrodewidth L and the electrode space S in the grating electrode of thepresent invention are set preferably to not smaller than λ/8 and notgreater than λ for the wavelength λ of a surface acoustic wave.Furthermore, to minimize the attenuation of the surface acoustic wavedue to the reflection from the grating electrode, it is set preferablyto λ/3n or λ/2n (n: positive integer). Since with an excess of increasein n, a fine patterning process of the electrode becomes difficult, n isset preferably to 8 or smaller.

Besides, in consideration of attenuation due to the reflection and aneasiness of electrode patterning technique, the electrode width L andthe electrode space S in the grating electrode of the present inventionare set still more preferably to λ/6. Besides, at the output electrodeabove or below the semiconductor layer or in the alternating portion ofa ground output electrode over the grating electrode outside thesemiconductor layer, the above electrode width and electrode space arepreferably made to further a half of it or less. Namely, the electrodewidth L and the space S between electrodes in the alternating portion ofthe above grating electrode with the above output electrode and theabove ground output electrode are preferably λ/16≦L≦λ/2 and λ/16≦S≦λ/2,respectively. For example, on setting the grating electrode width andelectrode space to λ/6 above the propagation path, the electrode widthand electrode space becomes λ/12 in the alternating portion of thegrating electrode and the output electrode on the semiconductor layer.Incidentally, as regards the arrangement of the alternating portion, thealternating portion of the output electrode and the grating electrodeare crossed preferably over all the semiconductor surface on the topface of the semiconductor layer.

Besides, the alternating width of the output electrode and the gratingelectrode is preferably narrower than the width of the propagation pathoutside above the propagation path and is set more preferably to 3λ. Thewavelength λ of a surface acoustic wave is expressed in terms of λ=v/f(v: velocity of a surface acoustic wave, and f: frequency) and since thevelocity v is publicly known for individual piezoelectric substratematerials, the width and electrode space of the grating electrode can bedetermined to a desired value, e.g., so as to satisfy λ/3n or λ/2n,corresponding to a frequency to be used.

The grating electrode of the present invention can be formed on the topor bottom of the semiconductor layer. In view of the crystallinity ofthe semiconductor layer, the more semiconductor film grows, the higherthe crystallinity becomes. In other words, according as going to thetop, the electron mobility increases. Accordingly, to raise theefficiency of interaction between a surface acoustic wave and electrons,it is preferable to allow interaction to occur at the top face of thesemiconductor layer.

In the present invention, formation of an output electrode and a groundoutput electrode across the grating electrode made it possible to takeout the convolution output in a lateral direction but not in verticaldirection of the semiconductor layer. As a result of this, the motion ofelectrons flows in a lateral direction in proportion to the size of thedepletion layer formed by the grating electrode, so that an effect ofenlarging the film thickness was implemented without reducing theresistance of the semiconductor layer. Furthermore, by optimizing theelectrode period of the alternating position or alternating portion ofthe grating electrode with the output electrode or the ground outputelectrode, a convolver has also attained a higher efficiency than ever.

Besides, the width W of the propagation path for a surface acoustic waveand the width a of the semiconductor film can be selected to anappropriate value, but matching the total resistance of thesemiconductor layer section and the grating electrode section to thesurface impedance of a surface acoustic wave enables the efficiency ofinteraction between the surface acoustic wave and electrons to beimproved. The total resistance value of the semiconductor layer sectionand the grating electrode section can be changed depending on the ratioW/a of the width W of the propagation path to that a of thesemiconductor layer. To acquire a higher efficiency, it is preferable toset W/a to more than 1 and more preferable to match the total resistanceof the semiconductor layer section and the grating electrode section tothe surface impedance of a surface acoustic wave. Empirically, amatching is easily made near W/a=8-10.

Forming of the buffer layer and the active layer of the presentinvention may be performed by any method, if it is a method for allowinga thin film to grow generally. For example, the evaporation process, themolecular beam epitaxy (MBE) process, the metalorganic molecular beamepitaxy (MOMBE) process and the metalorganic vapor phase deposition(MOCVD) process are especially preferred methods.

EXAMPLES

Referring now to specific examples, the present invention is describedbelow. However, the present invention is not limited to these examples.Using unidirectional electrodes to produce surface acoustic wavefunctional elements as practical devices makes it possible to reducelosses due to bidirectionality of surface acoustic waves.

Example 1

On a piezoelectric substrate 1, as a 128° Y-cut LiNbO₃ single-crystalsubstrate three inches in diameter, a buffer layer 2 ofAl_(0.5)Ga_(0.5)AsSb was grown to a thickness of 50 nm by the MBEmethod, and then an active layer 3 of InSb was grown to a thickness of500 nm. When electrical characteristics of the active layer weremeasured by the Van der Pauw method, its carrier density and electronmobility were found to be n_(o)=1.7×10¹⁶/cm³ and μ=33400 cm²/Vs,respectively. Then as shown in FIG. 1, the buffer layer 2 and activelayer 3 were etched by photolithography to be strips with a width of aso that the layers were only outside a propagation path for surfaceacoustic waves (its width is denoted by W). Next, the lift-off processwas used to form grating electrodes 7, extending across the propagationpath for surface acoustic waves and the active layer 3; a cascade inputelectrode 4 for surface acoustic waves; a cascade output electrode 5;and an electrode 6 for applying a direct-current electric field to theactive layer 3. The grating electrodes 7 were provided, with the width Lthereof being 0.5 μm, the spacing between the grating electrodes being0.5 μm, and the ratio of propagation path width to semiconductor layerwidth (W/a) being 10 (W=263 μm and a=26.3 μm). FIG. 1 shows thestructure of a surface acoustic wave amplifier produced through theprocess described above. FIG. 5 is an enlarged schematic view of thegrating electrodes on the propagation path and semiconductor layer. Whenamplification characteristics were measured at 1520 MHz using a networkanalyzer (8510B from Yokokawa Hewlett Packard), with a voltage of 3 Vapplied to the electrode 6, the amplification gain, that is, differencebetween gain after an electric field is applied and insertion lossbefore the electric field is applied, was found to be 29 dB. The valueof L and that of S were λ/6.

Comparative Example 1

By the MBE method, InSb film was grown to a thickness of 500 nm on apiezoelectric substrate 1, or a 128° Y-cut LiNbO₃ single-crystalsubstrate three inches in diameter. When electrical characteristics ofthe InSb film were measured at room temperature, its carrier density andelectron mobility were found to be n_(o)=2.0×10¹⁶/cm³ and μ=6500 cm²/Vs,respectively. Then when the amplification gain was measured using asurface acoustic wave amplifier, made in the same way as in the case ofExample 1, no amplification was observed at as low as three volts. Thatis, since no buffer layer was available in Comparative example 1, InSbfilm quality was not improved, so that electron mobility decreased.Diffusion of Li and O atoms from the LiNbO₃ substrate deteriorated InSbfilm quality because the InSb layer was grown directly on the LiNbO₃layer. In addition, a current leak layer was formed at the interfacebetween the piezoelectric substrate and the InSb layer. The current leaklayer is considered to have caused amplification performance todeteriorate. The AlGaAsSb buffer layer used in Example 1 is found toincrease crystalinity by placing InSb, crystal structure, and latticeconstant close to each other and prevent diffusion of Li and O atomsfrom an LiNbO₃ substrate.

Comparative Example 2

Using a 128° Y-cut LiNbO₃ single-crystal substrate three inches indiameter as a piezoelectric substrate 1, a buffer layer and an activelayer were grown in the same way as in the case of Example 1. Then thebuffer layer 2 and active layer 3 were etched by photolithography sothat a semiconductor layer is positioned on a propagation path forsurface acoustic waves. Next, the lift-off method was used as in thecase of Example 1 to form an input electrode 4, an output electrode 5,and an electrode 6 for applying a direct-current electric field to thesemiconductor layer. FIG. 6 is a schematic view of Comparative example2. When the amplification characteristics of a surface acoustic waveamplifier produced through the process described above were measured at1520 MHz, with a voltage of 3 V applied to the semiconductor layer ofthe amplifier, no amplification was observed. In this comparativeexample, surface acoustic waves must be generated through the bufferlayer and active layer, which formed thick, to cause interaction betweensurface acoustic waves and electrons in the semiconductor layer. InComparative example 2, the active layer was so thick, at 500 nm that theinteraction was inefficient.

Example 2

On a piezoelectric substrate 1, as a 128° Y-cut LiNbO₃ single-crystalsubstrate three inches in diameter, SiO₂ film 9 30 nm thick was formedby the sputtering method, then an Al_(0.5)Ga_(0.5)AsSb film was grown toa thickness of 50 nm as a buffer layer 2 by the MBE method, and next,InSb film, that is, an active layer 3, was grown to a thickness of 500nm. When the electrical characteristics of the active layer weremeasured in the same way as in the case of Example 1, its carrierdensity and electron mobility were found to be n_(o)=1.8×10¹⁶/cm³ andμ=31400 cm²/Vs, respectively. Then through the same process as in thecase of Example 1, a surface acoustic wave amplifier having the samestructure as the amplifier in FIG. 1 was made, with L, S, and W/a set to0.7 μm, 0.7 μm, and 10, respectively (W=400 μm and a=40 μm). The valuesof L and S were equal to λ/6. FIG. 7 is an enlarged schematic view ofthe propagation path and grating electrodes. When amplificationcharacteristics were measured at 1 GHz, with a voltage of 5 V applied tothe electrode 6, amplification is found to correspond to 28.8 dB. Thisin turn means that a large amplification gain is provided even if SiO₂film is formed on a piezoelectric substrate.

Comparative Example 3

As in the case of Example 2, on a piezoelectric substrate 1, as a 128°Y-cut LiNbO₃ single-crystal substrate three inches in diameter, SiO₂film was formed to a thickness of 30 nm, and then InSb film was formedto a thickness of 500 nm by the MBE method. The electron mobility of theInSb film was μ=5900 cm²/Vs. It was determined that inserting a bufferlayer of AlGaAsSb causes electron mobility to significantly increase.When a surface acoustic wave amplifier, made through the same process asin the case of Example 1, was used to measure amplificationcharacteristics in the same way as in the case of Example 2, noamplification was observed. In Comparative example 3, the SiO₂ layermade it possible to inhibit Li and O atoms from diffusing from thepiezoelectric substrate. However, because the InSb film was growndirectly on an amorphous SiO₂ layer, its quality was so low that noamplification was observed at practically low voltages. A comparisonwith Example 2 has shown that inserting a buffer layer of AlGaAsSbmarkedly increases electron mobility.

Example 3

On a piezoelectric substrate 1, as a 64° Y-cut LiNbO₃ single-crystalsubstrate three inches in diameter, a buffer layer 2 ofAl_(0.5)Ga_(0.5)AsSb was grown to a thickness of 50 nm, and then anactive layer 3 of InSb was grown to a thickness of 500 nm by the MBEmethod. When electrical characteristics of the active layer weremeasured in the same way as in the case of Example 1, its carrierdensity and electron mobility were found to be n_(o)=1.7×10¹⁶/cm³ andμ=33000 cm²/Vs, respectively. Next, through the same process as in thecase of Example 1, a surface acoustic wave amplifier having the samestructure as in FIG. 1 was made, with L, S, and λ/6 set to 0.75 μm andW/a set to 10 (W=300 μm and a=30 μm). When a voltage of 3 V was appliedto the electrodes for applying a direct-current electric field whichwere formed at both ends of the semiconductor, an amplification gain of35 dB was observed at 1 GHz. This indicates that selecting the bestpiezoelectric substrate material increases amplification.

Example 4

On a piezoelectric substrate 1, as a 128° Y-cut LiNbO₃ single-crystalsubstrate three inches in diameter, a first buffer layer 2A ofAl_(0.5)Ga_(0.5)AsSb was grown to a thickness of 50 nm by the MBEmethod. Then on the buffer layer, a second buffer layer 2B of InSb 200nm thick and a third buffer layer 2C of Al_(0.5)In_(0.5)Sb 100 nm thickwere deposited one on top of the other in that order. Next, on thebuffer layers, an active layer 3 of InSb was grown to a thickness of 200nm. When electrical characteristics of the active layer were measured inthe same way as in the case of Example 1, its carrier density andelectron mobility were found to be n_(o)=1.5×10¹⁶/cm³ and μ=34800cm²/Vs. Then through the same process as in the case of Example 1, asurface acoustic wave amplifier having the same structure as in FIG. 1was made, with L, S, and λ/6 set to 0.7 μm and W/a set to 8 (W=400 μmand a=50 μm). FIG. 8 is an enlarged schematic view of the propagationpath and grating electrodes of Example 4. When a voltage of 5 V wasapplied to the electrodes for applying a direct-current electric field,which were formed at both ends of the semiconductor, to measure theamplification characteristics of the surface acoustic wave amplifier,the amplification gain at 1 GHz was found to be 33 dB.

Example 5

By the laser abrasion method, piezoelectric film 15 of LiNbO₃ was grownto a thickness of 200 nm on a sapphire R-surface substrate 10 of 3inches in diameter to make a piezoelectric film substrate. Next, on thissubstrate, a buffer layer 2 of Al_(0.5)Ga_(0.5)AsSb was grown to athickness of 50 nm by the MBE method, and an active layer 3 of InSb wasgrown to a thickness of 500 nm. When electrical characteristics of theactive layer were measured in the same way as in the case of Example 1,its carrier density and electron mobility were found to ben_(o)=2.4×10¹⁶/cm³ and μ=25300 cm²/Vs, respectively. Then through thesame process as in the case of Example 1, a surface acoustic waveamplifier having the same structure as in FIG. 1 was made, with L, S,and W/a set to 0.8 μm, 0.8 μm, and 8, respectively (W=480 μm and a=60μm). FIG. 9 is a schematic view of the amplifier. When a voltage of 5 Vwas applied to the electrodes for applying a direct-current electricfield, which were formed at both ends of the semiconductor, to measurethe amplification characteristics of the surface acoustic wave amplifierof Example 5, an amplification gain of 19 dB was provided at 1 GHz. Thevalues of L and S were equal to λ/6.

Example 6

On a piezoelectric substrate 1, as a 128° Y-cut LiNbO₃ substrate, asemiconductor lower part output electrode 16 of Al 400 nm thick wasformed by vacuum deposition, and then a buffer layer 2 ofAl_(0.5)Ga_(0.5)AsSb was grown to a thickness of 50 nm over thesubstrate by the MBE method. Next, an active layer 3 of InSb was grownto a thickness of 500 nm. When electrical characteristics of the activelayer were measured at room temperature in the same way as in the caseof Example 1, its carrier density and electron mobility were found to ben_(o)=2.6×10¹⁶/cm³ and μ=25400 cm²/Vs, respectively. Then as shown inFIG. 1, the buffer layer 2 and active layer 3 were etched byphotolithography to be strips so that the layers were only outside apropagation path for surface acoustic waves. After the output electrodeof Al was removed by wet etching, two input electrodes (an inputelectrode and a reference signal input electrode) and grating electrodes7, extending across the propagation path for surface acoustic waves andactive layer, were formed by the lift-off method. The grating electrodes7 were shaped so that L=S=λ/6=1.4 μm and that W/a=10 (W=400 μm and a=40μm). Finally, a ground output electrode 18 was formed at the back of thepiezoelectric substrate. FIG. 10 shows the cross-sectional structure ofa surface acoustic wave convolver made through the above process. Whenthe convolution characteristics of an output with a frequency of 1 GHzfrom the semiconductor lower part output electrode 16 were measured withan oscilloscope by applying an input signal (1 mW) through one of theinput electrodes 4 and a reference signal (1 mW) through the other tothe element of the example, the convolution output was found to be agood non-linear signal and its efficiency to correspond to −39.5 dBm.

Example 7

A semiconductor layer similar to that in Example 6 was grown on apiezoelectric substrate, and a surface acoustic wave convolver was madein the same way as in the case of Example 6. The grating electrodes 7were shaped so that L=S=λ/6=3.3 μm and that W/a=10 (W=400 μm and a=40μm). When the convolution characteristics of an output with a frequencyof 400 MHz from the semiconductor lower part output electrode 16 weremeasured with an oscilloscope by applying an input signal (1 mW) throughone of the input electrodes 4 and a reference signal (1 mW) through theother to the element of the example, the convolution output was found tobe a good non-linear signal and its efficiency to be −39 dBm. Acomparison with Example 6 shows that a good convolution output can beprovided irrespective of frequency.

Comparative Example 4

On a 128° Y-cut LiNbO₃ substrate on which Al film was deposited, InSbfilm was grown to a thickness of 500 nm by the MBE method. Whenelectrical characteristics of the InSb film were measured, it was foundto have a low electron mobility of μ=6000 cm²/Vs. Evaluation of theconvolution characteristics of a surface acoustic wave convolver, madein the same way as in the case of Example 6, showed that the efficiencyof convolution output corresponded to only −51 dBm. This in turn meansthat if a semiconductor layer consisted of an InSb layer only, InSbcrystalinity is so low that interaction between surface acoustic wavesand electrons is inefficient.

Comparative Example 5

On a 128° Y-cut LiNbO₃ substrate, Al film was deposited in the same wayas in the case of Example 6. Next, a buffer layer 2 ofAl_(0.5)Ga_(0.5)AsSb was grown to a thickness of 50 nm by the MBEmethod, and then an active layer 3 of InSb was grown to a thickness of500 nm. The active layer was found to have the same electricalcharacteristics as in the case of Example 6. By photolithography, thesemiconductor layer was etched to be positioned on a propagation pathfor surface acoustic waves. After exposed Al film was etched, an outputelectrode 17 was formed on two input electrodes and a semiconductor bythe lift-off method. Finally, a ground output electrode 18 was formed atthe back of the piezoelectric substrate 1. FIG. 11 is a schematic viewof a surface acoustic wave convolver made according to the example. Whenconvolution characteristics were measured in the same way as in the caseof Example 6, the efficiency of convolution output was found tocorrespond to only −54 dBm. This means that if a semiconductor is on apropagation path, surface acoustic wave loss is so large thatinteraction between surface acoustic waves and electrons is inefficient.

Example 8

On a piezoelectric substrate 1, as a 128° Y-cut LiNbO₃ single-crystalsubstrate, Al film was deposited to form grating electrodes 7 in place,and then a buffer layer 2 of Al_(0.5)Ga_(0.5)AsSb was grown to athickness of 50 nm at a low temperature by the MBE method. Next, anactive layer 3 of InSb was grown to a thickness of 500 nm. Whenelectrical characteristics of the active layer were measured in the sameway, its electron mobility was found to be μ=25600 cm²/Vs. Byphotolithography, the buffer layer 2 and active layer 3 were etched tobe strips as shown in FIG. 12. Moreover, two input electrodes 4 and anoutput electrode 17 on a semiconductor layer were formed by the lift-offmethod. L=S=λ/6 =3.3 μm and W/a=10 (W=400 μm and a=40 μm). Finally, aground output electrode 18 was formed at the back of the piezoelectricsubstrate. When the convolution characteristics were measured forExample 8 in the same way as in the case of Example 7, the efficiency ofconvolution output was found to correspond to −40 dBm. The width of thegrating electrodes used for the example and the spacing between themwere λ/6.

Example 9

Using a piezoelectric substrate 1, as a 128° Y-cut LiNbO₃ single-crystalsubstrate three inches in diameter, a buffer layer 2 ofAl_(0.5)Ga_(0.5)AsSb was grown to a thickness of 50 nm by the MBEmethod, and then an active layer 3 of InSb was grown to a thickness of500 nm. The carrier density and electron mobility of the active layerwere n_(o)=1.8×10¹⁶/cm³ and μ=33000 cm²/Vs, respectively. Then as shownin FIG. 1, the buffer layer 2 and active layer 3 were etched bylithography to be strips, and grating electrodes 7, an output electrode19 intersecting the grating electrodes 7 on a semiconductor layer, andtwo input electrodes 4 were formed by the liftoff method. Finally, aground output electrode 18 was formed at the back of piezoelectricsubstrate. The width of the grating electrodes 7, L, and the spacingbetween them, S, were λ/6=4.0 μm. At the intersection on thesemiconductor, however, L=S=λ/12=2.0 μm and W/a=10 (W=400 μm and a=40μm). When the convolution characteristics were measured for Example 9 inthe same way as in the case of Example 7, a good convolution output witha frequency of 333 MHz was obtained from between the output electrode 19and the ground output electrode 18 at the back of the piezoelectricsubstrate, and the efficiency of the output was found to correspond to−35 dBm. That is, forming the output electrode strengthened interactionon the semiconductor layer, so that higher efficiency was provided,compared with the output electrode in Example 7. In Example 9, thegrating electrodes and output electrode intersecting them may be formedunder the semiconductor layer. In addition, the ground output electrodemay also be formed under the semiconductor layer.

Example 10

On a piezoelectric substrate 1, or a 128° Y-cut LiNbO₃ substrate, asemiconductor layer 20 consisting of a buffer layer 2 and an activelayer 3 was formed outside a propagation path in the same way as in thecase of Example 9. The active layer had the same film characteristics asthat of Example 9. In addition, SiO₂ film 21 300 nm thick was formed onthe semiconductor layer. Then by the lift-off method, grating electrodes7 and an output electrode 19 were formed on the semiconductor layer, andthen a ground output electrode 22, which was narrower than thepropagation path, was formed so that the electrode intersected thoseparts of the grating electrode which were opposite to the semiconductorlayer. FIG. 14 is a schematic view of a surface acoustic wave functionalelement according to Example 10. When the convolution characteristicswere measured for Example 10 in the same way as in the case of Example7, an excellent convolution output was obtained from the outputelectrode, and the efficiency corresponding to −32 dBm was attained. InExample 10, the width of the grating electrodes, L1, and spacing betweenthem, S1, on the propagation path, satisfied the equation 2L1=S1=λ/4=5μm. Example 10 was arranged so that the width of the grating electrodes,that of the output electrode, and the distance between the gratingelectrodes and the output electrode was λ/8 at the intersection.

Although a ground output electrode can be formed to be wider than apropagation path, the output electrode is more preferably arranged to benarrower than the propagation path and intersect grating electrodesoutside the path.

In Example 10, a dielectric layer 21 was inserted between thesemiconductor layer and the grating electrodes. The dielectric layer 21is for making schottky contacts between the semiconductor layer andgrating electrodes. However, the dielectric layer 21 is unnecessary ifschottky contacts can easily be formed by grating electrode deposition.

Example 11

A surface acoustic wave functional element was formed in the same way asin the case of Example 10 so that the width of grating electrodes 7 on apropagation path, L2, and the spacing between them, S2, satisfied theequation L2=S2=λ/8 and that the width of grating electrodes on thesemiconductor layer and at the intersections between grating electrodesopposite to the semiconductor layer, output electrode 19, and groundoutput electrode 22, L3, and the spacing between the electrodes, S3,satisfied the equation L3=S3=λ/16. For Example 11, the value of λ was 40μm. FIG. 15 is a general schematic view of the surface acoustic wavefunctional element, and FIG. 16 is an enlarged view of an intersection.The intersections between the grating electrodes 7 and the ground outputelectrode 24 were formed outside the propagation path so that the widthof the intersections, E, was smaller than that of the propagation path,W, and equal to 3λ. When the convolution characteristics were measuredfor Example 11 in the same way as in the case of Example 7, a goodconvolution output with a frequency of 200 MHz was obtained from theoutput electrode, and its efficiency was markedly high, at −30 dBm. FIG.17 shows a convolution output waveform observed.

Example 12

A surface acoustic wave functional element was made in the same way asin the case of Example 10. Connections between the grating electrodes 7and output electrode 19 were formed over the semiconductor andpropagation path as shown especially in FIG. 18. Example 12 was arrangedso that the width of the grating and output electrodes and the spacingbetween them were λ/8. When the convolution characteristics of a surfaceacoustic wave functional element were measured for Example 12 in thesame way as in the case of Example 7, a good convolution output wasobtained from the output electrode, and its efficiency corresponded to−36 dBm.

Example 13

A surface acoustic wave functional element was made in the same way asin the case of Example 10. As shown especially in FIG. 19, the length ofthe semiconductor was doubled, and deformed grating electrodes 23, anoutput electrode 19, and a ground output electrode 22 were formed toprevent the pitch between the grating and output electrodes fromchanging. Example 13 was arranged so that the width of the gratingelectrodes, L; the spacing between them; the width of the outputelectrode; and the distance between the grating electrodes and outputelectrode were all λ/8. When the convolution characteristics of asurface acoustic wave functional element were measured for Example 13 inthe same way as in the case of Example 7, a good convolution output wasobtained from the output electrode, and its efficiency corresponded to−36 dBm. The electrode structure according to Example 13 makes it easierto finely work the intersections between the grating and outputelectrodes.

Example 14

As in the case of Example 10, on a 128° Y-cut LiNbO₃ single-crystalsubstrate 1, a buffer layer 2 of Al_(0.5)Ga_(0.5)AsSb was grown to athickness of 50 nm by the MBE method, and then an active layer 3 of InSbwas grown to a thickness of 500 nm on the buffer layer. The InSb filmhad the same characteristics as that of Example 9. In Example 14,etching was done, leaving parts of the semiconductor layer in placeintact as shown in FIG. 20. Then the lift-off method was used to formtwo input electrodes 4, grating electrodes 7 on the semiconductor layer,and an output electrode 24, which extended over a propagation path whileintersecting the grating electrodes 7. Finally, a ground outputelectrode 18 was formed at the back of the piezoelectric substrate. Whenthe convolution characteristics of a surface acoustic wave functionalelement according to Example 14 were measured similarly, a goodconvolution output was obtained, and its efficiency corresponded to −40dBm.

Example 15

On a piezoelectric substrate 1, as a 128° Y-cut LiNbO₃ substrate, asemiconductor layer 20 of InSb was grown to a thickness of 500 nm by theMBE method. When the electrical characteristics of the semiconductorlayer were measured, its electron mobility was found to be μ=6500cm²/Vs. A dielectric layer 21, including SiO₂ film,.and strip dielectricfilm 25B which were 30 nm thick were formed on the semiconductor layer.By photolithography, the InSb layer was etched so that it was onlyoutside a propagation path. Moreover, by the lift-off method, gratingelectrodes 7 and output electrodes 4 were formed as shown in FIG. 21A.Then strip dielectric film 25A was formed on those parts of the gratingelectrodes which were opposite to the semiconductor layer, and an outputelectrode 26 was formed on the strip dielectric film 25A. Finally, atthe back of the piezoelectric substrate, a ground output electrode 18was formed in the position corresponding to the strip dielectric film.The width of the grating electrodes in Example 15, L, and the spacingbetween them, S, were L=S=λ/6=3.33 μm, and the width of the propagationpath, W, and that of the semiconductor, a, were W=400 μm and a=40 μm,respectively. When the convolution characteristics were measured forExample 15 in the same way as in the case of Example 7, a goodconvolution output was obtained from the output electrode on the stripdielectric film, and its efficiency corresponded to −39 dBm. That is, itwas confirmed that a convolution output signal due to interaction on asemiconductor layer between surface acoustic waves traveling throughgrating electrodes can be extracted efficiently through dielectric film.

The strip dielectric film 25 of the example does not always need to havea sandwich structure but may be formed only on top of the gratingelectrodes. The ground output electrode may also be formed underdielectric film provided at the bottom of the grating electrodes. It isalso possible to form the grating electrodes under the semiconductorlayer. Besides the output electrode on the dielectric film, an outputelectrode can be provided on top of, or at the bottom of, thesemiconductor layer to attain high efficiency by adding outputstogether.

Example 16

A film structure was built in the same way as in the case of Example 15.Etching was done so that an InSb layer 20, that is., a semiconductorlayer was only outside a propagation path. Then the lift-off method wasused as shown in FIG. 22 to form two input electrodes 4, gratingelectrodes 7, and an output electrode 19 which was disposed so that theelectrode intersect the grating electrodes 7 on the semiconductor layer.In addition, strip dielectric film 25 was formed in those parts of thegrating electrodes which were opposite to the semiconductor layer 20,and an output electrode 26 was formed on the film. Finally, at the backof the piezoelectric substrate, a ground output electrode 18 wasprovided in the position corresponding to the strip dielectric film 25.The width of the grating electrodes in Example 16, L, and the spacingbetween them, S, were L=S=λ/6 =3.33 μm, and the width of the propagationpath, W, and that of the semiconductor, a, were W=400 μm and a=40 μm,respectively. When the convolution characteristics were measured forExample 16 in the same way as in the case of Example 7, a goodconvolution output was obtained from the output electrode 26 on thestrip dielectric film and from the output electrode 19, and itsefficiency corresponded to −37 dBm. The grating and output electrodesmay be formed under the semiconductor layer. The grating electrodes mayalso be sandwiched between strip dielectric layers.

Example 17

On a piezoelectric substrate 1, as a 128° Y-cut LiNbO₃ single-crystalsubstrate, alternate grating electrodes 7 were formed by varyingalternately lengths in width of the grating electrodes in an appropriatecombination as shown in FIGS. 2 and 3. Then an InSb layer 20 was grownto a thickness of 500 nm by the MBE method. The electron mobility of thelayer was found to be μ=6000 cm²/Vs. Unnecessary parts of the InSb layerwere etched by photolithography to remove them, and strip dielectricfilm 25 was formed at one end of each of the grating electrodes as inExample 15. The lift-off method was used to form two input electrodes 4,an output electrode 17 on the semiconductor layer, and an outputelectrode 26 on the strip dielectric film. Finally, at the back of thepiezoelectric substrate, a ground output electrode 18 was provided inthe position corresponding to the strip dielectric film. When theconvolution characteristics were measured for Example 17 in the same wayas in the case of Example 7, a good convolution output was obtained fromthe two output electrodes on the strip dielectric film and thesemiconductor layer, and its efficiency corresponded to −38 dBm. Thewidth of the grating electrodes in Example 17, L, and the spacingbetween them, S, were L=S=λ/6=3.33 μm.

Example 18

To make a surface acoustic wave functional element in the same way as inthe case of Example 8, grating electrodes 7 were formed in part under asemiconductor layer 20 as shown in FIG. 24, and an output electrode 27was formed so that a gap G was provided between the output electrode andthe ends of the grating electrodes. Example 18 was arranged so that thewidth of the grating electrodes, L, and the spacing between them, S,were L=S=λ/6 and that G=λ/4. When the convolution characteristics of thesurface acoustic wave functional element of Example 17 were measured inthe same way as in the case of Example 7, a good convolution output wasobtained from the output electrode, and its efficiency corresponded to−40 dBm. The output electrode of the example causes the interactiveparts of the grating and output electrodes to shorten but eliminates theneed for intersections, thus facilitating fine work.

Example 19

Using a 36° Y-cut LiTaO₃ single-crystal substrate as a piezoelectricsubstrate, a surface acoustic wave functional element was made in thesame way as in the case of Example 11. When the convolutioncharacteristics of the element were measured similarly, a goodconvolution output was obtained from an output electrode, and itsefficiency corresponded to −39 dBm.

As described above, the surface acoustic wave convolvers in the examplesattained unprecedentedly high efficiency corresponding to values higherthan −40 dBm. Such high efficiency makes it possible to use convolversin various applications.

INDUSTRIAL APPLICABILITY

In making a surface acoustic wave functional element of the presentinvention, providing a buffer layer of the present invention on apiezoelectric substrate when a semiconductor layer was grown caused anactive layer of excellent film quality to be formed. Disposing thesemiconductor layer outside a propagation path for surface acousticwaves minimized surface acoustic wave loss. The width of gratingelectrodes and the spacing between them formed on the propagation pathcould be chosen so that reflection of surface acoustic waves werereduced. In addition, forming an output electrode and a ground outputelectrode which intersect the grating electrodes significantly increasedthe efficiency of interaction between surface acoustic waves andelectrons.

A surface acoustic wave functional element according to the presentinvention, when applied to a surface acoustic wave amplifier havingsemiconductors provided with direct-current application electrodes, canattain an unusually high amplification gain at practically low voltage.The element, when applied to a surface acoustic wave convolver, can alsoattain unprecedentedly high efficiency. In other words, a surfaceacoustic wave functional element of the present invention radicallyinnovates parts used in portable equipment for mobile communication andalone replaces an amplifier, a filter, or their peripheral circuits.Using a surface acoustic wave convolver according to the presentinvention as CDMA-related equipment in spread spectrum communication,whose future development is looked forward to, makes it possible toreduce power consumption and increase efficiency at the same time. Takentogether, the present invention offers unlimited industrialapplicability.

What is claimed is:
 1. A surface acoustic wave functional elementcomprising: an input electrode: an output electrode; a semiconductorlayer provided on a piezoelectric substrate and grating electrodes forconveying a surface acoustic wave to said semiconductor layer, whereinsaid semiconductor layer is situated outside above a propagation path ofa surface acoustic wave propagating from said input electrode and saidsemiconductor layer is composed of an active layer and a buffer layerlattice-matching thereto and a plurality of said grating electrodes areplaced on said propagation path perpendicularly to the propagationdirection and in a greater width than that of the propagation path.
 2. Asurface acoustic wave functional element as set forth in claim 1,wherein said semiconductor layer is thin enough to prevent the breakingof wire in said grating electrodes.
 3. A surface acoustic wavefunctional element as set forth in claim 2, wherein said semiconductorlayer is 500 nm thick or less.
 4. A surface acoustic wave functionalelement as set forth in claim 1, wherein one-end portions of saidgrating electrodes are formed on said semiconductor layer.
 5. A surfaceacoustic wave functional element as set forth in claim 1, comprising aplurality of grating electrodes with their width L satisfying L=λ/3n (n:positive integer) or L=λ/2n (n: positive integer) and theirinter-electrode space S satisfying S=λ/3n (n: positive integer) orS=λ/2n (n: positive integer) for the wavelength λ of a surface acousticwave propagating through said propagation path.
 6. A surface acousticwave functional element as set forth in claim 5, wherein the width L ofsaid grating electrodes satisfies λ/8≦L≦λ and the inter-electrode spaceS satisfies λ/8≦S≦λ.
 7. A surface acoustic wave functional element asset forth in claim 5, wherein the width L of said grating electrodessatisfies L=λ/6 and the inter-electrode space S satisfies λ/6.
 8. Asurface acoustic wave functional element as set forth in claim 1,further comprising: an electrode for applying a DC electric field tosaid semiconductor layer.
 9. A surface acoustic wave functional elementcomprising: an input electrode; a reference signal input electrode; asemiconductor layer provided on a piezoelectric substrate and gratingelectrodes for conveying a surface acoustic wave to said semiconductorlayer, wherein said semiconductor layer is situated outside above apropagation path of a surface acoustic wave propagating from said inputelectrode and said reference signal input electrode, said semiconductorlayer is composed of an active layer and a buffer layer lattice-matchingthereto, a plurality of said grating electrodes are placed on saidpropagation path perpendicularly to the propagation direction and in agreater width than that of the propagation path, and two input signalspropagating from said reference signal input electrode and said inputelectrode are subjected to convolution so as to be outputted from anoutput electrode and an output ground electrode.
 10. A surface acousticwave functional element as set forth in claim 9, comprising: acomb-shaped output electrode so arranged as to cross said gratingelectrodes and become equal in potential thereto.
 11. A surface acousticwave functional element as set forth in claim 10, wherein saidcomb-shaped output electrode is formed above the semiconductor layer.12. A surface acoustic wave functional element as set forth in claim 10,wherein said comb-shaped output electrode is formed below thesemiconductor layer.
 13. A surface acoustic wave functional element asset forth in claim 10, wherein the grating electrodes above thepropagation path differ in electrode period from the grating electrodesformed in alternating above or below the semiconductor layer and thecomb-shaped output electrode.
 14. A surface acoustic wave functionalelement as set forth in claim 9, comprising a uniform output electrodeat the bottom of said semiconductor layer; and a uniform ground outputelectrode at the bottom of said piezoelectric substrate.
 15. A surfaceacoustic wave functional element as set forth in claim 10, wherein acomb-shaped ground output electrode is disposed opposite to said gratingelectrodes and said comb-shaped output electrode.
 16. A surface acousticwave functional element as set forth in claim 15, wherein the electrodewidth L of an alternating portion of said grating electrodes with acomb-shaped output electrode or a comb-shaped ground output electrodesatisfies λ/16≦L≦λ/2 and the inter-electrode space S satisfiesλ/16≦S≦λ/2.
 17. A surface acoustic wave functional element comprising: asemiconductor layer; and a plurality of grating electrodes for conveyinga surface acoustic wave to the semiconductor layer provided on apiezoelectric substrate or on a piezoelectric film substrate, whereinsaid semiconductor layer is situated outside above a propagation paththrough which a surface acoustic wave propagates, said gratingelectrodes are formed perpendicularly to the propagating direction abovesaid semiconductor layer, a strip dielectric film is formed at the topof a portion of the grating electrodes opposed to said semiconductorlayer across said propagation path and an output electrode is formed onthe strip dielectric film.
 18. A surface acoustic wave functionalelement as set forth in claim 17, wherein said strip dielectric film isformed at the top and the bottom of said grating electrodes.
 19. Asurface acoustic wave functional element as set forth in claim 18,wherein an additional output electrode is formed below the stripdielectric film formed at the bottom of said grating electrodes.
 20. Asurface acoustic wave functional element as set forth in claim 17,wherein a uniform output electrode is formed at the bottom of saidsemiconductor layer.
 21. A surface acoustic wave functional element asset forth in claim 17, wherein said grating electrodes are formed belowsaid semiconductor layer.
 22. A surface acoustic wave functional elementas set forth in claim 21, wherein a uniform output electrode is formedat the top of said semiconductor layer.
 23. A surface acoustic wavefunctional element as set forth in claim 17, wherein said gratingelectrodes have a structure of lengths in width alternately varied in anappropriate combination and a strip dielectric film is formed atrespective one ends of alternate grating electrodes.
 24. A surfaceacoustic wave functional element as set forth in claim 17, wherein acomb-shaped output electrode is formed so as to cross said gratingelectrodes on said semiconductor layer and become the same potential.25. A surface acoustic wave functional element as set forth in claim 17,wherein the grating electrodes above the propagation path differ inelectrode period from the grating electrodes formed in alternating aboveor below the semiconductor layer and the comb-shaped electrode.
 26. Asurface acoustic wave functional element as set forth in claim 17,wherein the width L of said grating electrodes satisfies λ/8≦L≦λ and thespace S between the grating electrodes satisfies λ/8≦S≦λ.
 27. A surfaceacoustic wave functional element as set forth in claim 17, wherein thewidth L of said grating electrodes is λ/6 and the space S between thegrating electrodes is λ/6.
 28. A surface acoustic wave functionalelement as set forth in claim 17, wherein said semiconductor layer iscomposed of an active layer and a buffer layer lattice-matching thereto.29. A surface acoustic wave functional element as set forth in any oneof claims 1 and 17, wherein the ratio of the width W of the propagationpath for a surface acoustic wave to the width a of the semiconductorlayer is determined in such a manner as to almost equate the electricwave impedance of a surface acoustic wave to that of the semiconductorlayer.
 30. A surface acoustic wave functional element as set forth inany one of claims 1 and 17, wherein the ratio of the width W of saidpropagation path to the width a of said semiconductor layer satisfiesW/a>1.
 31. A surface acoustic wave functional element as set forth inany one of claims 1 and 17, wherein the ratio of the width W of saidpropagation path to the width a of said semiconductor layer satisfiesW/a=8-10.
 32. A surface acoustic wave functional element as set forth inany one of claim 1, wherein a semiconductor selected from a groupcomprising, InAs, InSb, and GaAs is employed for a semiconductor layer.33. A surface acoustic wave functional element as set forth in any oneof claims 1 and 17, wherein a substrate selected from a group comprisingLiNbO₃ single-crystal substrate, LiTaO₃ single-crystal substrate andKNbO₃ single-crystal substrate is employed for said piezoelectricsubstrate.
 34. A surface acoustic wave functional element as set forthin any one of claims 1 and 17, wherein a piezoelectric film substrateformed using a film selected from a group comprising LiNbO₃ film, LiTaO₃film, KNbO₃ film, PZT film and PbTiO₃ film is employed for saidpiezoelectric substrate.